A population study was undertaken in two health districts of children with severe mental handicap. Prevalence figures for behaviour problems are presented. A stratified random sample of 200 families was drawn from the population. Detailed structured interviews were carried out with the children's parents. The children were divided into two groups, comprising those with behaviour problems and those with no problems. There was a significant association between behaviour disorder and incontinence, lack of self-help skills, poor reading, writing and counting skills, and poor communication skills. Environmental correlates were few. There were no differences between groups in family size, birth order, age of parents, marital discord, social class or income. However, behaviour problems were more common in one-parent families. An association was found between maternal stress and problem behaviour. The possible mechanisms underlying this association are discussed.